Purdue Online Writing Lab
This site offers many useful handouts on writing, writing across the disciplines, usage, grammar and mechanics.
Rensselaer Writing Center
In addition to providing links to several other writing centers, the Rensselaer site offers several useful handouts on writing conventions across the disciplines. You will also find links to an online dictionary and thesaurus.
University of Illinois Writer's Workshop
This site offers resources for both students and instructors and provides useful information for ESL students and their teachers.